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Date:   Tue, 26 Feb 2019 14:16:28 -0800
From:   Hyun Kwon <hyun.kwon@xilinx.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC:     Hyun Kwon <hyunk@xilinx.com>,
        Stefano Stabellini <stefanos@xilinx.com>,
        Sonal Santan <sonals@xilinx.com>,
        Cyril Chemparathy <cyrilc@xilinx.com>,
        Jiaying Liang <jliang@xilinx.com>,
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Subject: Re: [PATCH RFC 1/1] uio: Add dma-buf import ioctls
Message-ID: <20190226221627.GA10631@smtp.xilinx.com>
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Hi Greg,

Thanks for the comments.

On Tue, 2019-02-26 at 03:53:11 -0800, Greg Kroah-Hartman wrote:
> On Sat, Feb 23, 2019 at 12:28:17PM -0800, Hyun Kwon wrote:
> > Add the dmabuf map / unmap interfaces. This allows the user driver
> > to be able to import the external dmabuf and use it from user space.
> > 
> > Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
> > ---
> >  drivers/uio/Makefile         |   2 +-
> >  drivers/uio/uio.c            |  43 +++++++++
> >  drivers/uio/uio_dmabuf.c     | 210 +++++++++++++++++++++++++++++++++++++++++++
> >  drivers/uio/uio_dmabuf.h     |  26 ++++++
> >  include/uapi/linux/uio/uio.h |  33 +++++++
> >  5 files changed, 313 insertions(+), 1 deletion(-)
> >  create mode 100644 drivers/uio/uio_dmabuf.c
> >  create mode 100644 drivers/uio/uio_dmabuf.h
> >  create mode 100644 include/uapi/linux/uio/uio.h
> > 
> > diff --git a/drivers/uio/Makefile b/drivers/uio/Makefile
> > index c285dd2..5da16c7 100644
> > --- a/drivers/uio/Makefile
> > +++ b/drivers/uio/Makefile
> > @@ -1,5 +1,5 @@
> >  # SPDX-License-Identifier: GPL-2.0
> > -obj-$(CONFIG_UIO)	+= uio.o
> > +obj-$(CONFIG_UIO)	+= uio.o uio_dmabuf.o
> >  obj-$(CONFIG_UIO_CIF)	+= uio_cif.o
> >  obj-$(CONFIG_UIO_PDRV_GENIRQ)	+= uio_pdrv_genirq.o
> >  obj-$(CONFIG_UIO_DMEM_GENIRQ)	+= uio_dmem_genirq.o
> > diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
> > index 1313422..6841f98 100644
> > --- a/drivers/uio/uio.c
> > +++ b/drivers/uio/uio.c
> > @@ -24,6 +24,12 @@
> >  #include <linux/kobject.h>
> >  #include <linux/cdev.h>
> >  #include <linux/uio_driver.h>
> > +#include <linux/list.h>
> > +#include <linux/mutex.h>
> > +
> > +#include <uapi/linux/uio/uio.h>
> > +
> > +#include "uio_dmabuf.h"
> >  
> >  #define UIO_MAX_DEVICES		(1U << MINORBITS)
> >  
> > @@ -454,6 +460,8 @@ static irqreturn_t uio_interrupt(int irq, void *dev_id)
> >  struct uio_listener {
> >  	struct uio_device *dev;
> >  	s32 event_count;
> > +	struct list_head dbufs;
> > +	struct mutex dbufs_lock; /* protect @dbufs */
> >  };
> >  
> >  static int uio_open(struct inode *inode, struct file *filep)
> > @@ -500,6 +508,9 @@ static int uio_open(struct inode *inode, struct file *filep)
> >  	if (ret)
> >  		goto err_infoopen;
> >  
> > +	INIT_LIST_HEAD(&listener->dbufs);
> > +	mutex_init(&listener->dbufs_lock);
> > +
> >  	return 0;
> >  
> >  err_infoopen:
> > @@ -529,6 +540,10 @@ static int uio_release(struct inode *inode, struct file *filep)
> >  	struct uio_listener *listener = filep->private_data;
> >  	struct uio_device *idev = listener->dev;
> >  
> > +	ret = uio_dmabuf_cleanup(idev, &listener->dbufs, &listener->dbufs_lock);
> > +	if (ret)
> > +		dev_err(&idev->dev, "failed to clean up the dma bufs\n");
> > +
> >  	mutex_lock(&idev->info_lock);
> >  	if (idev->info && idev->info->release)
> >  		ret = idev->info->release(idev->info, inode);
> > @@ -652,6 +667,33 @@ static ssize_t uio_write(struct file *filep, const char __user *buf,
> >  	return retval ? retval : sizeof(s32);
> >  }
> >  
> > +static long uio_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
> 
> We have resisted adding a uio ioctl for a long time, can't you do this
> through sysfs somehow?
> 

The dmabuf is managed as per process resource, so it's hard to do it through
sysfs.

> A meta-comment about your ioctl structure:
> 
> > +#define UIO_DMABUF_DIR_BIDIR	1
> > +#define UIO_DMABUF_DIR_TO_DEV	2
> > +#define UIO_DMABUF_DIR_FROM_DEV	3
> > +#define UIO_DMABUF_DIR_NONE	4
> 
> enumerated type?
> 
> > +
> > +struct uio_dmabuf_args {
> > +	__s32	dbuf_fd;
> > +	__u64	dma_addr;
> > +	__u64	size;
> > +	__u32	dir;
> 
> Why the odd alignment?  Are you sure this is the best packing for such a
> structure?
> 
> Why is dbuf_fd __s32?  dir can be __u8, right?

The dmabuf fd is defined as int, so __s32 seems correct. Please let me know
otherwise. The dir can be __u8. Will fix if there is v2 at all.

> 
> I don't know that dma layer very well, it would be good to get some
> review from others to see if this really is even a viable thing to do.
> The fd handling seems a bit "odd" here, but maybe I just do not
> understand it.

Agreed. So I'm looking forward to feedback or if there's more sensible
alternative.

Thanks,
-hyun

> 
> thanks,
> 
> greg k-h
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